Techniques to monitor and/or track a person's eye movements (for example, to detect a locus of the person's gaze) have been used in a variety of contexts. However, as described in more detail below, determining a locus of a person's gaze, however, does not provide insight into the quality of that person's level of attention, and therefore decisions based solely on the locus of a person's gaze may result in unsatisfactory man-machine interactions.